The primary system surfaces of water-cooled nuclear reactors and equipment develop a corrosion product oxide ("rust") film during normal operation. The film incorporates radionuclides from the circulating coolant into its lattice, and becomes radioactive. This contributes to the out-of-core radiation fields, increases personnel radiation exposure, and hinders inspection and maintenance. Thus, effective decontamination has to substantially remove the oxide film, with minimal corrosion and metal substrate effects.
Oxide removal depends upon the film's structure, which is a function of the coolant chemistry and the metal substrate. For boiling water nuclear reactors (BWR's), "oxidizing" conditions prevail (0.5-0.2 ppm O.sub.2), and the system alloys are 300 series stainless steels. These conditions result in a relatively thick, porous, hematite film, with iron as the predominant metal. Chromium is converted to chromates, and, hence, continually dissolves in the coolant. In contrast, pressurized water nuclear reactors (PWR's) operate with reducing water chemistry (&lt;0.0005 ppm oxygen), and the primary system contains a large fraction of high nickel alloys. These conditions produce a denser, more coherent and tenacious oxide film, containing chromium in a nickel ferrite lattice. Thus, BWR films are easier to dissolve and remove than PWR films; the latter usually require an oxidation treatment for chromium removal before the film can be dissolved. For either case, iron represents the dominant metal species in solution after film removal.
Commercially available decontamination solutions generally fall into three categories. These are the Citrox solutions, Can-Decon solutions and Low Oxidation State Metal Ion (LOMI) solutions such as are described in the processes discussed in "An Assessment of Chemical Processes for the Postaccident Decontamination of Reactor Coolant Systems" EPRI Report NP-2866 of February 1983. The first solution uses organic acid species only, such as the Citrox-like solutions, which contain organic acids that remove the oxide film by both dissolution and spallation mechanisms. Citric and oxalic acids are the usual components. These solutions are effective and ion exchange well, but produce particulates and have precipitated iron during plant applications. A second solution uses a chelant solution, such as the Can-Decon-like solutions which use chelants to avoid precipitation and reduce the particulate generation. However, the chelants usually depress the ion exchange parameters. A third solution is an LOMI solution which uses vanadium (II) in a picolinic/formic acid buffer. The vanadium (II) acts as a reductive dissolution agent on the oxide, and particulate generation is minimized. The principal drawbacks of these solutions are the inability to cation exchange the solution and the fact that vanadium can exist in multiple valence states.
As the oxide film dissolves, ferric iron (III) accumulates in solution. Iron (III) can induce base metal corrosion, intergranular attack (IGA) and intergranular stress crack corrosion (IGSCC); it can also behave as an oxidizing-type inhibitor and limit corrosion. For Citrox-like solutions, above 25 to 30 parts per million (ppm) of iron results in increased corrosion with IGA and IGSCC tendencies. The chelants in Can-Decon solutions form strong complexes with iron (III). Therefore, three behavorial regimes can be observed: (a) at 0 to 25 ppm iron (III), free corrosion with increased IGA/IGSCC tendencies, (b) at 25 to 130 ppm iron (III), reduced corrosion and IGSCC tendencies, but IGA may still occur; and (c) above approximately 130 ppm iron (III), Citrox-like behavior with increased corrosion. The dissolved iron (III) also depresses the dissolution kinetics The LOMI process removes the iron in the reduced, divalent state, and iron corrosion effects are minimized. However, after four to eight hours, the vanadium exists as the quadravalent species, and the solution behaves like an iron-containing Citrox solution.
Entire primary system decontamination is expected to result in dissolved iron concentrations of 100 to 200 ppm and last for about 20 to 96 hours. Thus, significant and deleterions iron (III)/metal effects upon corrosion, ion exchange and kinetics can be expected.